svn commit: r303747 - head/sys/vm

[Alan Cox]

Author: alc
Date: Thu Aug  4 16:20:12 2016
New Revision: 303747
URL: https://svnweb.freebsd.org/changeset/base/303747

Log:
  Clean up the comments and code style in and around vm_pageout_cluster().
  In particular, fix factual, grammatical, and spelling errors in various
  comments, and remove comments that are out of place in this function.
  
  Reviewed by:	kib, markj
  MFC after:	3 weeks
  Sponsored by:	EMC / Isilon Storage Division
  Differential Revision:	https://reviews.freebsd.org/D7410

Modified:
  head/sys/vm/vm_pageout.c

Modified: head/sys/vm/vm_pageout.c
==============================================================================
--- head/sys/vm/vm_pageout.c	Thu Aug  4 15:27:09 2016	(r303746)
+++ head/sys/vm/vm_pageout.c	Thu Aug  4 16:20:12 2016	(r303747)
@@ -355,41 +355,28 @@ vm_pageout_page_lock(vm_page_t m, vm_pag
 }
 
 /*
- * vm_pageout_clean:
- *
- * Clean the page and remove it from the laundry.
- * 
- * We set the busy bit to cause potential page faults on this page to
- * block.  Note the careful timing, however, the busy bit isn't set till
- * late and we cannot do anything that will mess with the page.
+ * Scan for pages at adjacent offsets within the given page's object that are
+ * eligible for laundering, form a cluster of these pages and the given page,
+ * and launder that cluster.
  */
 static int
 vm_pageout_cluster(vm_page_t m)
 {
 	vm_object_t object;
-	vm_page_t mc[2*vm_pageout_page_count], pb, ps;
-	int pageout_count;
-	int ib, is, page_base;
-	vm_pindex_t pindex = m->pindex;
+	vm_page_t mc[2 * vm_pageout_page_count], p, pb, ps;
+	vm_pindex_t pindex;
+	int ib, is, page_base, pageout_count;
 
-	vm_page_lock_assert(m, MA_OWNED);
+	vm_page_assert_locked(m);
 	object = m->object;
 	VM_OBJECT_ASSERT_WLOCKED(object);
+	pindex = m->pindex;
 
 	/*
-	 * It doesn't cost us anything to pageout OBJT_DEFAULT or OBJT_SWAP
-	 * with the new swapper, but we could have serious problems paging
-	 * out other object types if there is insufficient memory.  
-	 *
-	 * Unfortunately, checking free memory here is far too late, so the
-	 * check has been moved up a procedural level.
-	 */
-
-	/*
-	 * Can't clean the page if it's busy or held.
+	 * We can't clean the page if it is busy or held.
 	 */
 	vm_page_assert_unbusied(m);
-	KASSERT(m->hold_count == 0, ("vm_pageout_clean: page %p is held", m));
+	KASSERT(m->hold_count == 0, ("page %p is held", m));
 	vm_page_unlock(m);
 
 	mc[vm_pageout_page_count] = pb = ps = m;
@@ -399,33 +386,23 @@ vm_pageout_cluster(vm_page_t m)
 	is = 1;
 
 	/*
-	 * Scan object for clusterable pages.
-	 *
-	 * We can cluster ONLY if: ->> the page is NOT
-	 * clean, wired, busy, held, or mapped into a
-	 * buffer, and one of the following:
-	 * 1) The page is inactive, or a seldom used
-	 *    active page.
-	 * -or-
-	 * 2) we force the issue.
+	 * We can cluster only if the page is not clean, busy, or held, and
+	 * the page is inactive.
 	 *
 	 * During heavy mmap/modification loads the pageout
 	 * daemon can really fragment the underlying file
-	 * due to flushing pages out of order and not trying
-	 * align the clusters (which leave sporatic out-of-order
+	 * due to flushing pages out of order and not trying to
+	 * align the clusters (which leaves sporadic out-of-order
 	 * holes).  To solve this problem we do the reverse scan
 	 * first and attempt to align our cluster, then do a 
 	 * forward scan if room remains.
 	 */
 more:
-	while (ib && pageout_count < vm_pageout_page_count) {
-		vm_page_t p;
-
+	while (ib != 0 && pageout_count < vm_pageout_page_count) {
 		if (ib > pindex) {
 			ib = 0;
 			break;
 		}
-
 		if ((p = vm_page_prev(pb)) == NULL || vm_page_busied(p)) {
 			ib = 0;
 			break;
@@ -446,18 +423,16 @@ more:
 		mc[--page_base] = pb = p;
 		++pageout_count;
 		++ib;
+
 		/*
-		 * alignment boundary, stop here and switch directions.  Do
-		 * not clear ib.
+		 * We are at an alignment boundary.  Stop here, and switch
+		 * directions.  Do not clear ib.
 		 */
 		if ((pindex - (ib - 1)) % vm_pageout_page_count == 0)
 			break;
 	}
-
 	while (pageout_count < vm_pageout_page_count && 
 	    pindex + is < object->size) {
-		vm_page_t p;
-
 		if ((p = vm_page_next(ps)) == NULL || vm_page_busied(p))
 			break;
 		vm_page_test_dirty(p);
@@ -477,15 +452,12 @@ more:
 
 	/*
 	 * If we exhausted our forward scan, continue with the reverse scan
-	 * when possible, even past a page boundary.  This catches boundary
-	 * conditions.
+	 * when possible, even past an alignment boundary.  This catches
+	 * boundary conditions.
 	 */
-	if (ib && pageout_count < vm_pageout_page_count)
+	if (ib != 0 && pageout_count < vm_pageout_page_count)
 		goto more;
 
-	/*
-	 * we allow reads during pageouts...
-	 */
 	return (vm_pageout_flush(&mc[page_base], pageout_count, 0, 0, NULL,
 	    NULL));
 }